1. Field of the Invention
The present invention relates to single carrier type double-side polishing method and apparatus suitable for both-surface polishing of a semiconductor wafer, which is a raw material for a semiconductor device, and more particularly, to double-side polishing method and apparatus suitable for single wafer type polishing which processes one wafer with one carrier.
2. Description of the Related Art
As for double-side polishing of a semiconductor wafer, which is a raw material for a semiconductor device, there has been well used a planetary gear mechanism type double-side polishing apparatus. A planetary gear mechanism type double-side polishing apparatus is a kind of a batch type apparatus simultaneously polishing both surfaces of each of plural wafers. In a planetary gear mechanism type double-side polishing apparatus, plural carriers are inserted between rotatable upper and lower surface plates. The plural carriers are sufficiently smaller in diameter than the surface plates and are arranged around a rotation center of the surface plates while each holding one or plural wafers and each conducting a planetary motion in company with rotation of the surface plates. Thereby, the wafer held in each the carrier is polished on both surfaces thereof between the surface plates.
In recent years, semiconductor wafers subjected to both-surface polishing have rapidly increased in diameter to as large as 300 mm. It is expected to further increase a diameter in the future. In a case where such a large diameter wafer is polished on both surfaces thereof, a scale will be tremendously larger in a multicarrier type double-side polishing apparatus using plural carriers as in the case of the planetary gear mechanism type, leading to much of difficulty in securing a mechanical precision or suppressing an apparatus cost. In order to meet a high flatness required of a wafer, it is desired to alter processing conditions of each wafer. In consideration of these aspects, it has been generally understood that a single wafer type apparatus, which processes one wafer at a time, is advantageous for double-side polishing of a large diameter wafer.
The greatest feature in terms of a structure of a single wafer type double-side polishing apparatus is in that the apparatus is of the single carrier type using one carrier larger in outer diameter than the rotatable upper and lower surface plates. In the apparatus, one wafer smaller in diameter than the surface plates is held with the one carrier and the carrier is moved between the upper and lower surface plates in rotation to thereby polish both surfaces of the one large diameter wafer. Needless to say that the apparatus of this kind is smaller in size and is advantageous in the aspect of a price, as compared with the multicarrier type double-side polishing apparatus using plural carriers to simultaneously polish both surfaces of each of the plural wafers. One of such single wafer apparatuses is a “single wafer type double-side polishing apparatus” provided in JP-A 2001-315057.
In the “single wafer type double-side polishing apparatus” provided in JP-A 2001-315057, a carrier holds a wafer at a position eccentric from the center of the carrier. The carrier is arranged concentrically with respect to the upper and lower surface plates and is rotated about its center. The carrier rotates concentrically with respect to the surface plates, that is, makes concentric rotation, so that the wafer held eccentrically revolves around the center of the carrier and is polished on both surfaces thereof.
As a kind of a single carrier type using one carrier, which is not a single wafer type apparatus, provided in JP-A 2000-33559 is a batch type polishing apparatus in which plural wafers are held around the center of the carrier and, also, the carrier is arranged between the upper and lower surface plates eccentrically with respect thereto and moves circularly around the center of the surface plates.
Conventional single wafer type double-side polishing apparatuses including the apparatus provided in JP-A 2001-315057, however, have an intrinsic problem that a flatness of a wafer is harder to be secured from a polishing principle, as compared with a multicarrier type double-side polishing apparatus using plural carriers as in the case of the planetary gear type. The reason why is that in a case of a multicarrier type double-side polishing apparatus using plural carriers, the plural carriers are arranged in the outer peripheral portion of the upper and lower surface plates therebetween. When a carrier is disposed in the outer peripheral portion, a difference between peripheral speeds at the outer and inner sides is small. As a result, the wafers held in the carriers are polished at a comparatively uniform peripheral speed at every point on each of the wafers.
In a case of a single wafer type polishing apparatus, a wafer is slightly different in diameter from the surface plates, though the wafer is smaller in diameter than the surface plates. Therefore, one wafer is polished by the surface plates each using an area from a central portion to an outer peripheral portion thereof. In a case of the single wafer type polishing apparatus described in JP-A 2001-315057 in which the carrier arranged so as to be concentric with respect to the surface plates rotates about its center, a motion of the wafer held eccentrically with respect to the carrier is as shown in FIG. 5.
In FIG. 5, there are shown geometrical motion loci of the center of a 300 mm wafer, intermediate points spaced 75 mm (a half of the radius) in an eccentric direction and a direction opposite the eccentric direction from the center of the wafer and points on the outer edge spaced 150 mm (the radius) in the both directions from the center thereof. Note that though in actual polishing, the wafer rotates in the carrier, it is neglected in FIG. 5. An eccentricity of the wafer with respect to the carrier is 30 mm.
As seen from FIG. 5, in the case of the single wafer-type polishing apparatus described in JP-A 2001-315057 in which the carrier rotates about its center, the center of the wafer only rotates about the center of the surface plates at the same radius in the vicinity of the center of the surface plates. On the other hand, a point on the outer edge of the wafer in an eccentric direction only rotates about the center of the surface plates at the same radius along the outermost peripheral portion of the surface plates. The other points only rotate about the center of the surface plates at respective constant radii between those of the center and outer edge of the wafer. Herein, the peripheral speed of the center of the surface plates in rotation is 0. A peripheral speed of a point on the wafer increases as the point is farther from the center of the surface plates and finally reaches the maximum at a point on the outer edge of the wafer. As a result, a polishing rate at a point on the wafer by the surface plates is greatly different between the central portion and the outer peripheral portion of the wafer and no change occurs in each peripheral speed at a corresponding point on the wafer, leading to difficulty securing a flatness.
In actual polishing, the wafer rotates within the carrier and measures such as that a supply of a polishing liquid to the central portion is increased in order to supplement a difference between peripheral speeds, which prevents a flatness from decreasing to such an extent that would be otherwise expected, whereas even with such measures taken, it is hard to absorb the large difference in peripheral speed, resulting in difficulty securing a flatness.
In FIG. 6, there are shown geometrical loci in a case where the polishing apparatus shown in JP-A 2000-33559 is applied to single wafer type polishing. That is, the polishing apparatus shown in JP-A 2000-33559, which is of a single carrier type using one carrier, is of a batch type in which plural wafers are held in the carrier. In a case where it is assumed that one wafer is held in the carrier concentrically or eccentrically with respect to the center thereof, the carrier moves circularly around the center of the surface plates; therefore, the center of the wafer conducts a circular motion with a small radius corresponding to a circular motion of the carrier in the vicinity of the center of the surface plates. A point on the outer edge of the wafer conducts a circular motion with a small radius corresponding to a circular motion of the carrier in the outer peripheral portion of the surface plates. A point intermediate between both points conducts a circular motion with a small radius corresponding to a circular motion of the carrier in the intermediate portion of the surface plates. Note that, in this case, an eccentricity of the wafer with respect to the carrier is 10 mm and a radius of the circular motion of the carrier is 20 mm.
The polishing apparatus shown in JP-A 2000-33559 is basically the same as the single wafer type polishing apparatus described in JP-A 2001-315057 in that a peripheral speed of the surface plate is largely different according to a point on the wafer in a radial direction thereof, which results in a large difference in polishing rate, whereas the polishing apparatus shown in JP-A 2000-33559 is slightly more advantageous than the single wafer type polishing apparatus described in JP-A 2001-315057 in that points on the wafer alter distances from the center of the surface plates in company with the circular motion with a small radius. On the other hand, the polishing apparatus shown in JP-A 2000-33559 is more disadvantageous than the single wafer type polishing apparatus described in JP-A 2001-315057 in that radii of the motions of the points in a radial direction are small and a radius of a motion of a point in the wafer outer peripheral portion is especially small.